Chapter 3
REVIEW QUESTIONS
In the following problems, assume activities equal concentrations and ignore complexing
unless instructed otherwise. Use the constants in Table 3-1.
1. What is the pH of pure water in equilibrium with 1 atmosphere of carbon dioxide at
25℃?
2. What is the pH and calcium concentration of pure water in equilibrium with calcite and
one atmosphere of CO2 at 25℃?
3. What is the calcium concentration in a water in equilibrium with calcite at a Pco2 of
-2.5
10 atm at 50℃?
4. The water in question 3 is warmed from 50 to 25℃. How much calcite should precipitate,
assuming chemical equilibrium and (a) constant Pco2; (b) no loss or gain of CO2 gas? In case (b),
what .will be the final pH and Pco2?
5. A 0.01-m NaHCO3 solution is allowed to equilibrate with calcite at a Pco2 of 10-3.5atm at
250℃. How much Ca2+ (in ppm) will the solution contain, and what will be its pH?
6. A 0.01-m CaCl2 solution is allowed to eqtiilibrate with calcite at a Pco2of 10-3.5atm and
25℃. What will be the resulting pH and HCO~ concentration?
7. What should be the pH of a puddle of rain on an outcrop of dolomite at 25℃? (Assume
chemical equilibrium.)
8. An "ideal" rain consists of pure water in equilibrium with CO2 at a partial pressure of
-3.5
10 atm. By how many millimeters per century should limestone exposed to this rain be
dissolved if equilibrium is attained? Assume a rainfall of 1 m/year, 25℃, a system open to CO2,
and a density for calcite of 2.7 g/cm3.
9. If the rain in question 8 were "acid”, containing 0.1 meq/kg of free acidity, how many
millimeters per century would be dissolved?
10. A river water has the following chemical analysis:
Na+ 11.4 ppm SO4+ 58 ppm
K+ 1.56 ppm Alkalinity 1.30 meq/kg
2+
Ca 24.0 ppm pH 8.10
2+
Mg 10.0 ppm Temp. 10℃
-
CI 1.9 ppm
With what Pco2 would the water be in equilibrium? By how much is it supersaturated or
undersaturated with respect to calcite? (Calculate activity coefficients by the Debye-Huckel
equation,Chapter 2.)
11. Perform the same calculations as in Problem 10 using the WATEQ4F code. How do the
results compare to your "hand-calculated" values?